1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device having an improved dry etching step.
2. Description of the Related Art
In recent years, micropatterning of an element has advanced in accordance with the progress of the technique of a semiconductor integrated circuit, pattern dimensions having high accuracy has been demanded. A semiconductor integrated circuit can be generally obtained by stacking an insulating thin film such as a silicon oxide film having a predetermined pattern and a conductive thin film such as a polysilicon, aluminum, copper, tungsten, or silicide film on a semiconductor substrate such as a silicon substrate.
As techniques for processing the thin film into a predetermined pattern, lithographic, dry etching, and removing techniques are used. In the lithographic technique, after a photosensitive resist is coated on the thin film, the resist is exposed to a light beam or an ultraviolet beam in accordance with the predetermined pattern, and an exposed portion or non-exposed portion is selectively removed by development to form a resist pattern. In the dry etching technique, the underlying thin film is etched using the resist pattern as a mask. In the removing technique, the resist is removed.
However, as a degree of integration of semiconductor elements is increased, the required minimum size becomes small and dimensional precision of a pattern becomes more strict. Recently, a micropattern having a size of 0.5 .mu.m or less has been required. In order to cope with the above pattern in a small region, since the above techniques for forming a pattern have various problems, the techniques must be largely improved.
These problems are described below in detail.
As one method of processing an underlying thin film using a small resist pattern, an RIE method using a plasma is popularly used. According to this method, a substrate on which a target film is deposited is loaded in a vacuum vessel having a pair of parallel plate electrodes, and after the vessel is evacuated, a reactive gas having a halogen element or the like is supplied into the vessel. A plasma is produced by the gas using discharging caused by application of an RF power, and the target film is etched by the produced plasma.
According to this etching method, ions of various particles in the plasma are accelerated by a DC electric field generated at an ion sheath on the surfaces of the electrodes, and the ions having high energy are collided with the target film, thereby performing an ion-assisted chemical reaction. For this reason, etching is performed in the direction of the incident ions, and directional etching having no undercut can be performed.
Since all materials are excited or activated by this ion collision, differences between reactivities unique to materials cannot be easily obtained in this etching compared with etching using only radicals, and a ratio of etching rates of different materials, i.e., a selection ratio is generally low. For example, since an etching rate of a resist is high with respect to Al, it is difficult to form a pattern with high accuracy due to a large pattern conversion difference. In addition, since the thickness of a resist is small at a stepped portion, a wiring portion is disadvantageously etched to disconnect wiring lines.
In etching of a silicon oxide film, it has a low selection ratio of the silicon oxide film to an underlying material. That is, since underlying silicon (Si) or aluminum (Al) has a high etching rate, etching cannot be immediately stopped with high precision when the surface of the underlying material is exposed. For this reason, when contact holes having different depths are to be formed by etching, silicon or aluminum serving as the underlying layer at the bottom of a shallow hole is undesirably etched in a considerable amount, thereby degrading device characteristics.
In such dry etching, since the moving directions of radicals are not aligned with each other, an increase in etching rate ratio (selectivity) of a film to be etched to a mask at a desired constant etching rate of the film to be etched makes it impossible to form a highly precise pattern due to undesirable side-etching or deposition of the resultant pattern.
Anisotropic etching free from side-etching, an increase in etching rate ratio (selectivity) of the film to be etched to the mask, and a high etching rate of the film to be etched have a trade-off relationship. That is, it is difficult to simultaneously satisfy these three conditions.
In recent years, a mechanism for maintaining and controlling a wafer temperature to a low temperature of 0.degree. C. or less during etching is employed. Etching can be performed at a high etching rate by an ion-assisted reaction in a direction of a depth, and anisotropic etching can be performed in a lateral direction while the reaction is "frozen". Low-temperature wafer control allows control of a reaction on side walls of a pattern, so that the pattern shape can be controlled. For example, in etching of a silicon oxide film (SiO.sub.2), Oiwa (Dry Process Symposium P. 105, 1990) proposed etching of a tapered silicon oxide film (SiO.sub.2) within an appropriate range between the pressure and the substrate temperature.
Technical specifications required for a contact hole along with an increase in integration of semiconductor elements are a decrease in hole diameter and an increase in hole depth. When the hole diameter is decreased and the hole depth is increased, the diameter of the bottom of the hole can be made smaller than that defined in the device specifications because the side wall of the contact hole is tapered. A contact hole is a hole for electrically connecting underlying silicon and a wiring layer formed on the silicon oxide film. For this purpose, a metal (e.g., aluminum or tungsten) or polysilicon is buried in the contact hole. It is thus known that a more perfect electrical connection can be achieved when a contact area between the metal or polysilicon to be buried and the underlying silicon is increased. In view of improvement of electrical characteristics and an increase in integration density, the side wall of the contact hole must be vertical. That is, the specifications required for forming a contact hole used in a highly integrated device are a high selection ratio (at least 20) to silicon and a vertical side wall.
In a silicon oxide film, although it is possible to form a vertical side wall of a contact hole at a high selection ratio to silicon and a high substrate temperature, a resist pattern is thermally deformed at a substrate temperature of 160.degree. C. or more. Therefore, the upper limit of the taper angle of the side wall of the pattern is 83.degree.. It is therefore impossible to obtain a desired pattern with high precision. In ion milling of Al, Au, or Pt, since high-energy particles bombard against the substrate, the temperature of the substrate is increased during etching. The resist pattern is thermally deformed to disable high-precision etching.
Use of a silicon oxide or nitride film as a heat-resistant mask to etch copper or the like at high temperatures is reported. In this case, since copper tends to be oxidized at high temperatures, a residue may be produced, the shape is deformed, or diffusion of copper into the mask material occurs. Therefore, the electrical characteristics are degraded, and it is impossible to form a good wiring layer.
In etching of tungsten or the like, the etching rate of a peripheral wafer portion is different from that of a central wafer portion. When an area having a low etching rate is completely etched, overetching occurs in an area having a high etching rate. An underlying material is etched in a considerable amount, and the pattern shape is undesirably changed. When the size of the wafer is increased, it is impossible to form a desired pattern on the entire surface of the wafer.
In addition, when an insulating thin film such as a silicon oxide film is used as a mask, in an etching method using a plasma, ions and electrons in the plasma are incident on a thin film to be etched. The ions and electrons incident on the thin film cause charges to be accumulated in the thin film (charge-up). For example, when electrons are incident on a mask pattern from a diagonal direction, since the electrons are collided on any one of the right and left walls, amounts of charges to be accumulated in the right and left walls of the mask pattern are different from each other. An electric field newly generated due to asymmetry of the charge amounts in the right and left directions of the walls acts on ions to curve the movement direction of the ions, thereby degrading the anisotropy of the shape of the mask pattern. The micropattern cannot be easily etched with high accuracy.
When a metal material, especially AlSiCu or the like, is to be etched, after a resist film serving as an etching mask is removed and left to stand in the air, corrosion occurs in the metal material. Device characteristics are degraded, and a highly reliable device cannot be easily formed.
The following problems are posed by anisotropic etching of target substrates in accordance with conventional reactive ion etching techniques.
(1) It is impossible to etch a silicon oxide film to obtain a vertical side wall at a high selection ratio to the silicon oxide film.
(2) The etching rate of the central wafer portion is different from that of the peripheral wafer portion in a refractory metal film as of tungsten, a refractory metal silicide film, or a refractory metal oxide film when the size of the wafer is increased, thereby disabling uniformity on the entire wafer.
(3) Since a dry etching selection ratio of the etching mask to a material to be etched is low in reactive ion etching, the thickness of the etching mask material is largely reduced during etching. In addition, when the temperature of the target substrate is increased, the mask pattern is degraded due to a low heat resistance of the mask material. Therefore, high-precision etching cannot be performed.
(4) Assume that copper or the like is etched at a high temperature. Since copper tends to be oxidized at high temperatures, a residue is produced, the shape is degraded, and diffusion of copper to the mask material occurs. As a result, the electrical characteristics are degraded, and excellent wiring layers cannot be obtained.
(5) When an organic thin film is used as a mask material, since the thin film contains an impurity such as fluorine (F), the impurity is mixed in a plasma during the reactive ion etching to maintain the Al, Al alloy, or Si thin film. Especially, corrosion caused by this contamination occurs, and a highly reliable device cannot be obtained.
(6) When a mask material is an organic material or comprises an insulating thin film such as a silicon oxide thin film, the mask pattern is charged up by an amount of charge stored in the mask due to a balance between electrons and ions incident on the thin film in a plasma. The incident direction of the ions are bent, and a micropattern cannot be formed with high precision.
(7) It is often impossible to remove a mask material without damaging a material to be etched and materials adjacent to the material to be etched due to a combination of the mask material and the material to be etched and a combination of the material to be etched and adjacent materials.
(8) While the required minimum size of a pattern is decreased and the required dimensional precision becomes more strict along with an increase in integration degree of semiconductor elements, a micropattern having a size of 0.5 .mu.m or less is recently required. When an underlying thin film having a high reflectance, such as a polysilicon film and an aluminum film is to be patterned, light or an ultraviolet ray passing through the resist is reflected by the surface of the thin film during exposure, and a resist portion except other than the predetermined pattern is undesirably exposed to degrade the dimensional precision.
In order to solve the above problem, a method of forming a micropattern using a carbon film mask is proposed in, e.g., Published Unexamined Japanese Patent Application No. 58-212136. According to this method, a carbon film excellent in etching resistance is formed on the film to be etched. A resist is applied to the carbon film, and a resist pattern is formed by a conventional lithographic means. The carbon thin film is etched by reactive ion etching using this resist pattern as a mask. The resist is selectively removed from the carbon film by using an organic solvent to form a mask consisting of only the carbon thin film pattern. Reactive ion etching is performed using the carbon film pattern as a mask, thereby forming a thin film to be etched. Etching having a high selection ratio can be performed.
The carbon film formed on the film to be etched serves as an anti-reflection film, as described above, and an etching mask having resistance to dry etching.
In the step of forming the carbon film mask pattern, the organic resist is removed in an organic solvent, a solution mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2, or a solution obtained by adding H.sub.2 O thereto. When the material to be etched consists of Al as a major constituent, and the resist is removed by the solution mixture of H.sub.2 SO.sub.4 and H.sub.2 O.sub.2, the material to be etched itself is also etched.
Even if an organic solvent is used, the photo-cured resist or the like cannot be perfectly removed. An alkaline organic solvent or the like has a limited number of types of thin films to be etched because a metal material such as Al may be etched or corroded.
In the process using a solution, a lot of problems are posed in view of solution management and safety measures in operations. Therefore, this process is not suitable for the process for manufacturing semiconductor elements in a dry state.
On the other hand, dry ashing is available to cause an oxygen plasma to remove an organic resist. According to this method, a sample having an organic resist film is placed in a barrel or flat parallel palate type discharge reaction chamber, and oxygen gas is discharged to remove the organic resist film. According to this method, as compared with the method using the solution, the resist can be easily removed, and an underlying material may be a metal. The type of underlying material is not limited to a specific one. However, according this dry ashing method, since the sample is placed in a discharge to obtain a predetermined removal or etching rate required in practical use, both the organic resist and the carbon film are undesirably etched. It is therefore impossible to remove the organic resist with high selectivity to the carbon film.